Evaluation of thermal properties of ThO₂-based MOX fuels: an alternative to UO₂-based MOX fuels

This investigation systematically evaluates thermal properties (thermal expansion, thermal conductivity and melting temperatures) of (Th,U)O₂ or (Th,Pu)O₂ mixed oxides (MOX) using a two-pronged experimental and atomistic simulations methodologies. ThO₂-based MOX is a potential alternative fuel which can substitute UO₂-based MOX in currently operating reactors. Our combined MD simulations and high temperature XRD measurements indicate that incorporation of PuO₂ and UO₂ in ThO₂ systematically increases coefficient of thermal expansion. The thermal conductivity of the ThO₂ rich (Th_1-xPu_x)O₂ and (Th_1-xU_x)O₂ solid solutions was found to be highly non linear as a function of composition. Deviations from the perfect end member compositions introduced phonon scattering due to the nonuniform cation sublattice, thereby reducing the phonon mean free path and the thermal conductivity of the system. The degradation of thermal conductivity was more significant for (Th_1-xPu_x)O₂ than (Th_1-xU_x)O₂ due to the large lattice parameter mismatch of the end members. The MD calculated melting temperatures of ThO₂, UO₂ and PuO₂ using two-phase simulations, lie between 3650-3675 K, 3050-3075 K and 2800-2825 K, respectively, which match well with experiments. The MD calculated melting temperatures of (Th,V)O₂ and (Th,Pu)O₂ MOX show good agreement with the ideal solidus line in the high thoria section of the phase diagram and an evidence for a minimum is identified around 5 atom% of ThO₂ in the phase diagram of (Th,Pu)O₂ MOX. (author)